Electric utilities must be prepared to meet the demand for electrical power at all times. Load profiles for most utilities indicate a cyclic load, with demand for electric power typically varying with the time of the day, day of the week, and month of the year. Based upon historical load profile data, anticipated electrical demand can be predicted with a reasonable degree of certainty. The minimum electrical power demand is referred to as the base load and demand above the base load is referred to as peaking load. It is not uncommon for peaking loads to be double the base load during some periods of the load profile.
To meet peaking load demand, some power plants are load-cycled to follow large percentage load changes, while the base load is supplied by other plants running continuously at their rated megawatt output. Large fossil-fueled steam power plants, traditionally used for supplying base load, are increasingly being used to meet peaking power demand due to the use of less expensive power from nuclear units and power from cogeneration units to provide the base load.
Steam turbines used in electrical power plants typically have been designed for optimal operation in a particular range of steady state conditions. Cycling a steam turbine to meet peaking load can induce large thermal stresses in both the turbine and the boiler as a result of steep steam-to-metal temperature gradients that develop during rapid loading or unloading of the turbine. These elevated stress levels reduce equipment service life and increase maintenance costs. Turbine rotor stress is the most critical of these stresses and maximum rotor stress limits are often used to define the fastest load cycling rates.
Current cycling control systems for steam turbines do not use stress minimization as an explicit objective and generally operate the turbine at the rotor stress limit to provide good load following. Further, conventional turbine controllers typically rely on algorithms that optimize turbine load following and performance over only a single load change as opposed to over an extended load profile that includes several load changes. Such conventional controllers typically rely either on adjusting steam supply to the turbine (e.g., controlling a throttle valve to the turbine), or alternatively, on changing boiler pressure as the means for controlling generated power.
It is advantageous to operate the steam turbine system in a manner that both provides good load tracking and that minimizes stress on the system components. It is also desirable to operate the system in a manner to most efficiently convert the energy consumed by the thermal source to electrical energy.
It is thus an object of this invention to provide a steam turbines generator control system that provides cyclic turbine control with good load following and stress minimization.
It is a further object of this invention to provide a steam turbine-generator control system that provides economical long term cyclic turbine operation with the use of extended time horizons for prediction of turbine loading profiles.